Expediting spectral measurement in semiconductor device fabrication

ABSTRACT

A device and method for expediting spectral measurement in metrological activities during semiconductor device fabrication through interferometric spectroscopy of white light illumination during calibration, overlay, and recipe creation.

BACKGROUND OF THE INVENTION

Typically, spectral information used in metrological activitiesassociated with semiconductor device manufacture is acquired throughrelatively time consuming spectral measurements during calibration,overlay activities, recipe creation, and other metrological activities.Spectral measurements many times increase overall processing timethereby reducing process efficiency. Therefore, there is a need toexpedient acquisition of robust spectral information enabling a widerange of metrological activities in a manner preserving fabricationspeed and efficiency.

SUMMARY OF THE INVENTION

According to the teachings of the present invention there is provided aninterferometric-spectroscopy metrology tool including an interferometricmicroscope configured to create an interferogram from light beamsderived from white light illumination during vertical conveyance of afocal lens, the interferogram captured by a single-pixel detectorassociated with the interferometric microscope; and a computerconfigured to render the interferogram into a frequency spectrum throughapplication of a Fourier transformation on the interferogram.

According to a further feature of the present invention, the light beamsinclude a beam reflected from a broadband reflector so as tocharacterize a spectrum of the white light illumination in theinterferogram.

According to a further feature of the present invention, the broad bandreflector is implemented as a mirror.

According to a further feature of the present invention, the broad bandreflector is implemented as bare silicon.

According to a further feature of the present invention, the light beamsinclude a beam reflected from a processed wafer so as to characterize aproduct of a spectrum of white light illumination and a spectrum ofspectral reflectivity of the processed wafer in the interferogram.

According to a further feature of the present invention, theinterferometric microscope includes a Linnik beam splitter cube.

There is also provided according to the teachings of the presentinvention, a method of expediting acquisition of spectral data insemiconductor device fabrication metrology; the method including:

-   -   splitting white light illumination into two light beams with an        interferometric microscope;    -   receiving an interferogram on a single-pixel detector, the        interferogram formed from recombination of the light beams; and        applying a Fourier transform to the interferogram so as to        render the interferogram into a frequency spectrum        characterizing the white light illumination.

According to a further feature of the present invention, there is alsoprovided reflecting one of the light beams off a broadband reflector.

According to a further feature of the present invention, the broadbandreflector is implemented as a mirror.

According to a further feature of the present invention, the broadbandreflector is implemented as a bare wafer.

There is also provided according to the teachings of the presentinvention, splitting white light illumination into a reference lightbeam and a test light beam with the interferometric microscope;

-   -   capturing an interferogram on the single-pixel detector during        focusing of the white light illumination during overlay        sequence, the interferogram formed from recombination of the        light beams, the test light beam reflected from a processed        wafer having microstructures; and applying a Fourier transform        to the interferogram so as to render the interferogram into a        composite frequency spectrum characterizing both the white light        illumination and spectral reflectivity of the processed wafer.

According to a further feature of the present invention, there is alsoprovided dividing the composite frequency spectrum by the frequencyspectrum of the white light illumination so as to generate a frequencyspectrum characterizing a reflectivity of the processed wafer.

There is also provided according to the teachings of the presentinvention, an interferometric-spectroscopy metrology tool including aninterferometric microscope configured to create an interferogram fromwhite light illumination, the microscope having, a horizontallyconveyable reference mirror or a conveyable beam splitter cube, a focallens fixed at a focal distance from a processed wafer; atwo-dimensional, pixel-array detector configured to capture multiple,pixel-specific interferograms in accordance with a changing optical pathdistance of light beams derived from the white light illumination; and acomputer configured to apply a Fourier transform on each of thepixel-specific interferograms so as to generate a pixel-specific,frequency spectrum associated with each corresponding area of theprocessed wafer.

According to a further feature of the present invention, the computer isfurther configured to construct a synthetic image of at least one chosencentroid wavelength and respective chosen bandwidth.

According to a further feature of the present invention, the computer isfurther configured to apply a metric to the synthetic image.

According to a further feature of the present invention, the metric isselected from the group consisting of overlay metric Region ofInvestigation (ROI) of the processed wafer, average reflectivity, 3S,contrast, and target asymmetry.

There is also provided according to the teachings of the presentinvention, a method of expediting acquisition of hyperspectral data insemi-conductor device fabrication metrology, the method includingsplitting white light illumination with an interferometric microscopeinto two light beams; changing an optical path distance traveled by thelight beams while maintaining focus on the processed wafer; capturing apixel-specific interferograms on a two-dimensional, pixel-array detectorin accordance with the changing optical path distance traveled by thelight beams; applying a Fourier transformation to each pixel-specificinterferogram so as to generate a pixel-specific, frequency spectrum ofthe processed wafer; and assigning pixel, grey levels proportional to achosen pixel, centroid frequency and bandwidth.

According to a further feature of the present invention, the changingthe optical path distance is implemented through horizontal conveyanceof the reference mirror.

According to a further feature of the present invention, the changingthe optical path distance is implemented through conveyance of the beamsplitter.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.In regard to the invention, constituent components and theirconfiguration and features, method of operation, objectives, andadvantages are most clearly understood with reference to the followingdetailed description in view of the accompanying drawings in which:

FIG. 1 is a schematic diagram of an imaging overlay tool employing aninterferometric, imaging microscope, implemented with a single-pixeldetector according to an embodiment;

FIG. 2 is an interferogram captured by the imaging overlay tool of FIG.1, according to an embodiment;

FIG. 3A is a frequency spectrum derived from a Fourier transformation ofthe interferogram of FIG. 2, according to an embodiment;

FIG. 3B is a frequency spectrum measured with a spectrometer depictingthe substantial correspondence with frequency spectrum derived fromFourier transformation of the interferogram of FIG. 3A; according to anembodiment;

FIG. 4 is a flow chart of a method of expediting acquisition of spectraldata while focusing in an overlay sequence, according to an embodiment;

FIG. 5 is a schematic diagram of an imaging overlay tool employing aninterferometric, imaging microscope, implemented with a pixel-arraydetector, according to an embodiment;

FIG. 6 is an interferogram set of pixel-specific interferograms capturedby the imaging overlay tool of FIG. 5, according to an embodiment;

FIG. 7 is a frequency spectrum set derived from Fourier transformationof each of the pixel-specific interferograms of FIG. 6, according to anembodiment;

FIG. 8 is a flow chart of a method of expediting hyperspectral imagingof a processed wafer, according to an embodiment; and

FIG. 9 is a schematic diagram of a combinationinterferometric-microscope overlay and hyperspectral imaging tool,according to an embodiment;

It should be appreciated that figure elements are not drawn to scale andfor the sake of clarity, corresponding elements in various figures areidentically labeled.

DETAILED DESCRIPTION

In the following description, numerous details are set forth tofacilitate a clear understanding of the invention and it should beappreciated that the present invention may be practiced without thesespecific details. Furthermore, well-known methods, components andprocedures are omitted to highlight the present invention.

The following terminology will be used through this document:

“Metrological activities” includes and not limited to any one orcombination metrological tests:

“Overlay metrology” (OVL) refers to a technique for evaluating alignmentbetween upper and lower layer patterns most effectively evaluated atcertain wavelengths.

The OVL variance is calculated as the difference between centers ofsymmetry of inner and outer overlay targets.

“Kernel3S” refers to a metric that is three times the variance of theOVL calculated by three different overlying parts of the same overlaytargets. It should be as small as possible while high values mayindicate bad portability, variation over the target, bad recipe.

“Phase linearity” refers to a metric indicating variance of the OVL asgiven by the contribution of different harmonics.

“Through focus” is a metric implemented as overlay variation as afunction of focus position.

“Contrast precision” refers to contrast between targets and is afunction of the degree of measurable, high contrast possessed by gratingstructure image.

It should be appreciated that pixel-specific interferograms or spectraare associated with corresponding wafer sites.

The present invention is an interferometric-spectroscopy metrology toolemployed in semiconductor device fabrication. In one embodiment, thetool is implemented as an overlay tool configured to identify optimalfocus position during overlay sequence and in another embodiment thetool is implemented as a hyperspectral imaging tool for facilitating inrecipe creation and other metrics.

In reference to FIGS. 1 and 2, FIG. 1 is a schematic depiction ofinterferometric-spectroscopy, metrology tool including a computer 6linked to an interferometric, imaging microscope 1 having a beamsplitter cube 2, a focus cube 3, a horizontally-conveyable, referencemirror 4, a single-pixel, focus detector 5, vertically-conveyable, focallens 7, horizontal focus lens 15, and a tube lens 19.

During vertical conveyance of focal lens 7 in direction Z in an overlaysequence, white light illumination 8 is split by beam splitter cube 2into horizontal beam 12 and directed to reference mirror 4 and avertical light beam 13 and directed to broad band reflector 9. Reflectedlight beams 12 and 13 are recombined at beam splitter cube 2 as is knownin the art. Recombined beam 14 is directed to focus cube 3 through tubelens 19 and then to single pixel detector 5, according to an embodiment.When the optical path difference (OPD) between the horizontal andvertical light beams 12 and 13 approaches zero, a computer 6 linked tosingle-pixel detector 5 generates an interferogram as a plot ofintensity counts as a function of OPD in micrometers as depicted in FIG.2. A best focal point is identified as the maximum amplitude point fromthe envelope of the interferogram. Computer 6 renders interferogram intoa frequency spectrum through application of a Fourier transform as isknown to those skilled in Fourier Transform Spectroscopy (FTS) as setforth in “Introductory Fourier Transform Spectroscopy” by Robert JohnBell, for example.

FIG. 3A depicts a frequency spectrum of white light spectrumcharacteristic of the interferogram of FIG. 2 after Fouriertransformation and FIG. 3B is a frequency spectrum obtained throughdirect measurement through spectral measurements. A comparison of thetwo normalized spectra shows the semblance of the FTS derived spectrumwith the measured spectrum. The deviation between the spectra of FIGS.3A-3B is a result of the responsivity of single pixel detector 5employed in the FTS spectrum being non-normalized with the detectoremployed in the spectrometer.

Returning to FIG. 1, as shown vertical light beam 13 is first directedto a broad band reflector 9 implemented as either a mirror or a barewafer. The resulting spectrum of the white light illumination may beused to calibrate the illumination spectrum.

After a calibration spectrum is obtained, the process is repeated forprocessed wafer 20 so as to generate composite spectrum that is aproduct of the white light illumination spectrum and the spectralresponsivity of processed wafer 20.

FIG. 4 depicts a flow chart for expediting acquisition of spectral dataduring overlay sequence and is divided into two stages; a calibrationstage 30 and an acquisition stage 34, according to an embodiment. Asshown, at step 31 white light illumination and vertical beam isreflected from a broad band reflector 9, as noted above. At step 32 aninterferogram is captured, at step 33 the interferogram is transformedinto a frequency or wavelength spectrum of the white light illumination.Acquisition stage 34 begins at step 35 in which additional white lightillumination is again split into two beams and a vertical beam isdirected to processed wafer 20 as depicted in FIG. 1. In step 36, aninterferogram is captured while focusing lens 7 and holding referencemirror 4 in a single position during overlay sequence. In step 37 aFourier transformation of the interferogram is performed to produce acomposite spectrum of the illumination and wafer reflectivity. In step38, the composite spectrum is rendered into a reflectivity spectrum bydividing the composite spectrum by the illumination spectrum, as isknown in the art.

When the interferometric-spectroscopy metrology tool is implemented asan overlay tool it advantageously leverages existingLinnik-beam-splitter interferometer by widening its use to providespectral measurement of illumination sources and wafer reflectivity inthe absence of a spectrometer. When the target is a broad bandreflector, the tool can be used to calibrate the illumination sourcetool, as noted above, and when the target is implemented as a processedwafer, the tool facilitates generation of spectral reflectivity mapsuseful in the assessment of process variation and recipe creation.Furthermore, the derived spectral frequency data can be advantageouslyused to correct skewed overlay measurements resulting from deviation ofwafer process parameters like film thickness, refractive indexes, forexample.

Additionally, the required interferometric data is acquired duringfocusing in overlay sequences with no impact on OVL measurement time.

FIG. 5 depicts an embodiment of the interferometric-spectroscopymetrology tool implemented as a hyperspectral imaging tool configured tosynthesize images in accordance with centroids ofinterferometrically-derived reflectivity spectra.

The tool includes a horizontally-conveyable, interferometric microscope1 h employing a pixel-array detector (camera) 10, a verticallyconveyable, beam-splitter cube 2, a focus cube 3, ahorizontally-conveyable, reference mirror 4, a focal lens 7 held at afocal distance from a target, horizontal focus lens 15, a tube lens 19,a computer 6 h linked to pixel-array detector 10 and configured torender each pixel-specific interferogram into a pixel-specificreflectivity spectrum. In a certain embodiment computer 6 h is furtherconfigured to synthesize a synthetic image proportional to chosencentroid wavelength and chosen bandwidth on the basis of thereflectivity spectra. It should be appreciated that computer 6 hincludes all necessary input and output equipment.

FIG. 6 depicts an interferogram set 40 of multiple, pixel-specificinterferograms 42, captured by camera 10, according to an embodiment.Each specific interferogram captured by camera 10 corresponds to aspecific site on the target wafer on the wafer plane in an opticalimaging system thereby providing robust spectral information at eachcorresponding site of the wafer.

FIG. 7 depicts a spectrum set 50 of multiple, pixel-specificreflectivity spectra 52, after Fourier transformation of each of thepixel-specific interferogram of FIG. 6, according to an embodiment.Accordingly, each specific spectrum provides robust spectral informationat each corresponding site of the wafer.

FIG. 8 depicts a flow chart of the operational steps employed by thehyperspectral imaging tool of FIG. 6 in the syntheses of varioussynthetic images, according to an embodiment.

In step 61, white light illumination 8 is split by beam splitter 2, instep 62 the optical path distance of beams 12 and 13 is modulated whilemaintaining focus on processed target wafer 20. The optical pathdistance of beams 12 and 13 can be modulated through various schemes. Ina certain embodiment, light optical path distance modulation is achievedby conveying beam splitter 2 either horizontally or vertically, or, inanother embodiment, conveying reference mirror 4 while holding beamsplitter 2 in the same position, whereas in another embodiment, opticalpath modulation is achieved through a combination of these two methods.

In step 64, a set of pixel-specific interferograms 40, as shown in FIG.6, is captured by camera 10 from recombined light beams 12 and 13. Eachpixel of camera 10 corresponds to a wafer site such that each capturedinterferogram corresponds to the specific location of wafer from whichthe light 12 reflected, as noted above.

In step 67, computer 6 h applies a Fourier transform to eachpixel-specific interferogram 42 of the interferogram set 40 so as torender them into spectrum set 50 of pixel-specific frequency spectra 52,as is known in the art.

In step 68, pixel grey levels are assigned that are proportional to achosen pixel, centroid frequency and chosen bandwidth. It should beappreciated that in a certain embodiment, a plurality of pixel centroidsis employed simultaneously to provide desired resolution. It should beappreciated that the grey levels in certain embodiments are implementedas color levels or intensity levels.

In step 69, the pixel, grey levels of the pixel-array detector aredisplayed in preparation for use with the appropriate metric.

The application of interferometric spectroscopy in hyperspectral imagingof processed wafers advantageously provides significant time savings byreducing the customary, multiple spectral measurements into a singleinterferometric measurement that after rendering into spectrum providesrobust spectral information at every location of the wafer. Thisspectral information may be utilized during research and development tocreate a recipe of optimal operational parameters for customeroperation, to determine appropriate overlay parameters, and to providevarious wavelength viewing options in accordance with the wavelengthmost suitable for a chosen metrology tool.

FIG. 9 depicts an embodiment of combination interferometric-spectroscopymetrology tool implemented with both a camera 10 and a single-pixeldetector 5 in addition to the above-described hardware andconfigurations. The combination interferometric-spectroscopy metrologytool advantageously providing either overlay tool or hyperspectralimaging functionality in accordance with the metrology needs.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

What is claimed is:
 1. An interferometric-spectroscopy metrology toolcomprising: an interferometric microscope configured to create a firstinterferogram from light beams derived from a first white lightillumination during vertical conveyance of a focal lens, the firstinterferogram captured by a single-pixel detector associated with theinterferometric microscope, wherein the light beams include a beamreflected from a broadband reflector so as to characterize a spectrum ofthe first white light illumination in the first interferogram; and acomputer configured to render the first interferogram into a frequencyspectrum through application of a Fourier transformation on the firstinterferogram; wherein the interferometric microscope is furtherconfigured to create a second interferogram from a second white lightillumination during focusing of the second white illumination duringoverlay sequence, the second white light illumination comprising areference light beam and a test light beam, the test light beamreflected from a processed wafer having microstructures, the secondinterferogram captured by the single-pixel detector from therecombination of the reference light beam and the test light beam; andwherein the computer is further configured to render the secondinterferogram into a composite frequency spectrum characterizing thesecond white light illumination and the spectral reflectivity of theprocessed through application of a Fourier transformation on the secondinterferogram.
 2. The tool of claim 1, wherein the broadband reflectoris implemented as a mirror.
 3. The tool of claim 1, wherein thebroadband reflector is implemented as bare silicon.
 4. The tool of claim1, wherein the interferometric microscope includes a Linnik beamsplitter cube.
 5. A method of expediting acquisition of spectral data insemiconductor device fabrication metrology; the method comprising:splitting a first white light illumination into two light beams with aninterferometric microscope; reflecting one of the two light beams off abroadband reflector; receiving a first interferogram on a single-pixeldetector, the first interferogram formed from recombination of the twolight beams; applying a Fourier transform to the first interferogram soas to render the first interferogram into a frequency spectrumcharacterizing the first white light illumination; splitting a secondwhite light illumination into a reference light beam and a test lightbeam with the interferometric microscope; capturing a secondinterferogram on the single-pixel detector during focusing of the secondwhite light illumination during overlay sequence, the secondinterferogram formed from recombination of the reference light beam andthe test light beam, the test light beam reflected from a processedwafer having microstructures; and applying a Fourier transform to thesecond interferogram so as to render the second interferogram into acomposite frequency spectrum characterizing both the second white lightillumination and spectral reflectivity of the processed wafer.
 6. Themethod of claim 5, wherein the broadband reflector is implemented as amirror.
 7. The method of claim 5, wherein the broadband reflector isimplemented as a bare wafer.
 8. The method of claim 7, furthercomprising dividing the composite frequency spectrum by the frequencyspectrum of the first white light illumination so as to generate afrequency spectrum characterizing a reflectivity of the processed wafer.